1) Field of the Invention
The present invention relates to a method and instrument for measuring a preload of a rolling bearing. The method and instrument are useful for the easy, prompt and accurate determination of a preload applied to a double-row ball bearing or duplex ball bearing or a double-row roller bearing or duplex roller bearing.
2) Description of the Related Art
For example, a double-row ball bearing such as that shown in FIG. 5 or a duplex ball bearing 6 such as that depicted in FIG. 6 has conventionally been assembled in bearing portions of various machines and apparatuses. Of these, the double-row ball bearing 1 shown in FIG. 5 is constructed of an outer ring 3 having double-row outer raceways 2,2 on an inner peripheral wall thereof, an inner ring 5 having double-row inner raceways 4,4 on an outer peripheral wall thereof, and plural balls 12 disposed for rotation between the outer raceways 2,2 and the inner raceways 4,4. Based on rolling of these balls 12, a member, such as a housing, with the outer ring 3 internally fitted thereon and supported thereon and another member, such as a shaft, with the outer ring 5 externally fitted and supported thereon are allowed to rotate relative to each other.
The duplex ball bearing 6 illustrated in FIG. 6 is constructed by combining, in opposite directions, a pair of ball bearings 11,11 each of which is composed of an outer ring 8 having an outer raceway 7 on an inner peripheral wall thereof, an inner ring 10 having an inner raceway 9 formed on an outer peripheral wall thereof and plural balls 12 provided for rotation between the outer raceway 7 and the inner raceway 9.
In such a double-row ball bearing 1 or duplex ball bearing 6, a line a extending through the points of contact of each ball 12 with the outer raceway 2 or 7 and inner raceway 4 or 9 is tilted at a (contact) angle .alpha. with respect to a line b extending at a right angle relative to a central axis of rotation and, at the same time, each ball 12 is pressed between the outer raceway 2 or 7 and the inner raceway 4 or 9, namely, the double-row ball bearing 1 or duplex ball bearing 6 is applied with a preload. Since the degree of such a preload (hereinafter simply called "preload") significantly affects the performance of the double-row ball bearing 1 and the duplex ball bearing 6, it is necessary to control the preload at a desired value. In particular, high-performance ball bearings require strict control of the preload.
As disclosed in Japanese Patent Application Laid-Open (Kokai) No. SHO 58-196318 and Japanese Patent Publication (Kokai) No. HEI 2-61700, there have hence been proposed methods for measuring a preload, which has been applied to a ball bearing, by applying a load or vibrations in a thrust direction to a part of the ball bearing.
Of these, the method disclosed in Japanese Patent Application Laid-Open (Kokai) No. SHO 58-196318 comprises applying an axial load to a part of a ball bearing and, at the same time, measuring displacements corresponding to variations in load, whereby a preload is estimated from the value of the load at which an abrupt displacement begins to take place. In the case of the method disclosed in Japanese Patent Publication (Kokai) No. HEI 2-61700, vibrations are applied to a ball bearing by a vibrator to detect the resonance frequency of the ball bearing and a preload is then determined from the resonance frequency. In addition, it is also practiced to estimate a preload, which has been applied to a ball bearing, by applying rotary force to an outer ring or inner ring constructing the ball bearing and then measuring a starting torque upon commencement of rotation of the member applied with the rotary force.
When a preload of a rolling bearing such as a ball bearing is measured by such conventional methods as referred to above, inconvenience are encountered as will be described next.
First, in the case of the method disclosed in Japanese Patent Application Laid-Open (Kokai) No. SHO 58-196318, an axial load is applied to a part of a ball bearing. As a result, impression or damage may occur on the surfaces of raceways so that rotation accuracy and service life of the ball bearing may be impaired. A limitation is therefore imposed on the range in which the method can be used. Since the preload is estimated from the value of the load at the time point where an abrupt displacement begins to take place, it is difficult to precisely determine the preload. It has however been conducted to determine the rigidity of a bearing by applying a load in a range not forming impression or damage on the surface of a raceway and then measuring a resulting displacement.
Next, in the case of the method disclosed in Japanese Patent Publication (Kokai) No. HEI 2-61700, the measuring instrument becomes complex, for example, a vibrator is needed because vibrations have to be applied to a ball bearing. Since measurement is conducted by applying vibrations while making a ball bearing unable to rotate (i.e., while maintaining the ball bearing in a stopped state), it is impossible to make evaluation at a practical revolution speed or a revolution speed close to the practical revolution speed.
Further, in the case of the method in which a preload is measured based on a starting torque, the preload cannot be determined accurately. The starting torque is prone to be affected by not only the preload but also other factors such as the quantity and viscosity of a lubricant such as grease, the state of contact of seals, etc. Since it is difficult to precisely ascertain influence of these other factors to the starting torque, the preload cannot be determined accurately from the starting torque.